Wells that lack sufficient formation pressure to flow fluid in commercial quantities to the surface utilize some type of artificial lift. One type of artificial lift employs an electrical pump that is lowered into the well for producing the well fluid. The pump is typically a rotary pump driven by a submersible electrical motor. The pump may be a centrifugal type having a large number of stages of impellers and diffusers. Alternately, the pump may be of another rotary type, such as a progressing cavity pump. Submersible rotary pumps generally are referred to herein as “ESP's”.
Typically, an ESP is secured to the lower end of a string of production tubing made up of joints of pipe secured together by threads. The tubing is lowered into the well along with the pump, and the power cable to the motor is strapped alongside the tubing. Normally the well is cased and has perforations that allow well fluid to flow into the casing. The intake of the pump is in communication with the well fluid in the casing, and the discharge of the pump is into the tubing.
One disadvantage of an ESP installed on production tubing is the time and equipment needed to install and retrieve a tubing supported ESP. It is not uncommon to pull an ESP for repair or replacement every year and a half or so, depending upon the type of well fluid and operating conditions.
Although not common, techniques are known in the prior art for installing an ESP such that the ESP could be retrieved without pulling a string of tubing. An ESP cannot be suspended on conventional ESP power cable, which lacks adequate strength to support its own weight and the weight of an ESP in a well. Special strengthening techniques must be employed. For example, one type of installation employs coiled tubing to support the weight of the pump. Coiled tubing comprises metal, continuous tubing that is deployed from a large reel of a coiled tubing injector. Normally the pump discharge does not lead to the interior of the coiled tubing, because if so, the coiled tubing would need a fairly large diameter, which would require a larger coiled tubing injector and greater expense for the coiled tubing. If the cable is installed within the coiled tubing, the pump may discharge into the casing surrounding the coiled tubing if the casing is in good condition. The casing may have holes or cracks that cause leakage of the well fluid into the surrounding environment, particularly if the casing is in an old well. This leakage could cause contamination of fresh water zones. If the casing leaks, it is known that the operator could install a liner in the casing to prevent such occurrence.